1. Field of Invention
The present invention relates to an optical line-of-sight communication apparatus for coding, transmitting, receiving and decoding digital message bits.
2. Description of Prior Art
Heretofore, in nearly all prior art systems, message coding in line-of-sight optical communications systems was effected by temporally modulating the total intensity of the transmitted beam. There are two principle disadvantages to this process: (1) The turbulent atmosphere in the propagation path also modulates the beam in space and time, causing errors in the detection process and (2) the presence of aerosols along the optical path causes some of the optical energy to be scattered out of the beam. This off-axis scatter can be observed at wide angles to the initial direction of beam propagation and by monitoring the irradiance fluctuations, the message being transmitted can be decoded. Thus, the message can be intercepted by unauthorized parties to whom the message was not directed.